This invention relates generally to process instruments used in industrial process control systems. More particularly, the present invention relates to pressure transmitters having redundant mechanical and electronic remote seal systems.
Pressure transmitters may be used to remotely monitor the pressure of a process fluid. The pressure transmitter includes circuitry that conditions signals for a pressure sensor and transmits an output to a remote location where it can be monitored as representing the magnitude of the pressure. Remote seals, or remote diaphragm assemblies, are often used to distance the pressure transmitter from hazardous measurement environments, or for linking the pressure transmitter with inconveniently located process fluids. For example, remote seals are often used with corrosive or high temperature process fluids such as in chemical plants or oil refineries.
Typically, in those situations, a mechanical remote seal having a diaphragm assembly and a capillary tube can be used to couple the pressure transmitter to the process fluid while the pressure transmitter is located a safe distance away. These hydraulic remote seal systems communicate with the process fluid through a thin, flexible diaphragm, which is used to isolate the process fluid from a fill fluid used in the capillary tube. As the diaphragm flexes, the incompressible fill fluid translates pressure change through the capillary tube to a diaphragm located in the pressure transmitter. Deflection of the pressure transmitter diaphragm is transmitted through another fill fluid to a pressure sensor, which produces a signal relating to the pressure of the process fluid.
Capillary tubes can extend up to twenty-five meters in order to couple the pressure transmitter with the process fluid, which contributes to several drawbacks of mechanical, capillary tube remote seals. For example, lengthy capillary tubes produce slow response times, and numerous connection points have the potential to leak fill fluid resulting in inaccurate sensing. Also, for differential pressure readings it is necessary to use two remote seals, in either a balanced or tuned configuration. For differential pressure sensing, two remote seals relay two pressures to a differential pressure sensor in the transmitter. In balanced configurations, where two equal length capillary tubes are used in order to equalize back pressure, one of the capillaries typically is longer than necessary for the application. This is both costly and can produce additional uncertainty in the pressure readings. In tuned configurations, where the transmitter is calibrated to reconcile the back pressures created in the unequal length capillaries, temperature variations in the work environment can cause drift in the calibration, also adding uncertainty to the pressure readings.
Electronic remote seal systems have also been proposed in which the remote seal is in electronic communication with the process transmitter. With these configurations, a gage pressure sensor is positioned on the remote seal and the sensor output signal is transmitted to the process transmitter, either through a wire-based or wireless electronic communication system. For differential pressure readings, the remote seal gage pressure is then compared with another gage pressure detected either at the pressure transmitter or another remote seal. The pressure transmitter circuitry then calculates the differential pressure. Thus, electronic remote seals eliminate the drawbacks associated with capillary tubes. There are, however, drawbacks in calculating differential pressure from two gage pressures, rather than directly sensing differential pressure with a differential sensor. In differential pressure sensors, common mode errors, such as temperature effects or line pressure errors, are largely cancelled out because each pressure is centrally sensed. When comparing two gage pressures, the common point of reference is lost and each pressure reading introduces its own errors.